Interesting remarks, but:
Why adding these resistors has removed the hum ?
How come a mains lttle over or under voltage has a dramatic effect about hum ?
Why adding these resistors has removed the hum ?
How come a mains lttle over or under voltage has a dramatic effect about hum ?
Wasn't it just 😀 Also all the broken derelict stuff that suddenly appeared with them saying "but it was alright before..."
I think in this case the higher voltage just starts to push the relatively small transformer into the first stages of saturation. Also if you look at the circuit a couple of the rectified rails have 10,000uF reservoir caps which will give the transformer a hard time with very high current but short duration charging pulses. The resistors literally just take the edge off it.
This is off the web but shows the small physical size.
Mains can get as high as 253V in Europe now (230V +/-10%). And on sunny windy Sundays it will often get close to that limit here in the Netherlands with solar and wind energy peaking during low demand hours.
I've seen several older devices with open primaries on the transformers recently. Out of curiosity I did some measurements on random equipment and found 220/230 rated coils simply run into to saturation around 245V. Current draw shoots up there. A transformer that runs hot or humms is a good sign there's trouble.
A resistor works perfectly fine for low power devices. Just yesterday installed a resistor in series with an old Philips turntable motor that was running way too hot, after a few minutes on 250V it got too hot to touch. It was drawing close to 20W in stead of the rated 6W. 560 ohm in series gave a 25V voltage drop and a cool motor running at 4,5Watts.
For higher power devices a 24V transformer used as buck transformer is a more elegant solution, the problem is there is seldom room to install it internally.
I've seen several older devices with open primaries on the transformers recently. Out of curiosity I did some measurements on random equipment and found 220/230 rated coils simply run into to saturation around 245V. Current draw shoots up there. A transformer that runs hot or humms is a good sign there's trouble.
A resistor works perfectly fine for low power devices. Just yesterday installed a resistor in series with an old Philips turntable motor that was running way too hot, after a few minutes on 250V it got too hot to touch. It was drawing close to 20W in stead of the rated 6W. 560 ohm in series gave a 25V voltage drop and a cool motor running at 4,5Watts.
For higher power devices a 24V transformer used as buck transformer is a more elegant solution, the problem is there is seldom room to install it internally.
That's interesting, thanks.
I think that is the takeaway point really. Low power and I would say where the current draw is a fairly constant value under all operating conditions.
I haven't checked regularly but right now our AC is right on 230V next to the distributor panel. I believe the expected demand at this time of day (sunday around 20.45) should be kind of high? Would that cause the voltage to sag?
Could be interesting to log locally but I guess I can find something real time online for my area.
Interesting discussion on the effects of solar and wind. Makes me wonder, how would something like a PS Audio Power Plant handle this? Not that I'm ever likely to own one but still.. Something like it might be needed someday for vintage gear, if I understand correctly?
Could be interesting to log locally but I guess I can find something real time online for my area.
Interesting discussion on the effects of solar and wind. Makes me wonder, how would something like a PS Audio Power Plant handle this? Not that I'm ever likely to own one but still.. Something like it might be needed someday for vintage gear, if I understand correctly?
Always make sure the battery in your multimeter isn’t failing and causing a higher reading.
Had seen some high numbers here a while back before discovering the issue…
Had seen some high numbers here a while back before discovering the issue…
For a consistent load like a MD player this works very well. If the resistors are cold even better.
Yes and no. It's correct that the EU voltage spec was harmonized from 220 V and 240 V to 230 V, but the tolerances were changed such that the old equipment could still be used on the "new" voltage. I thought that was a rather elegant solution.
It sounds like the transformer in the MD player was saturating on the voltage peaks. Addressing that was prudent.
Tom
Yes, it is unthinkable, LOL. You can measure the voltage of that not-connected point on the transformer against the primary common. If it is higher than the input voltage you have a transformer with 220-240V taps. And connecting the 240V tap should improve the situation. If it is not tapped, there are a dozen of other, better options to reduce the mains voltage to the standby transformer. But none are as simple as the unthinkable series resistors. So if it works for you, it works.
You have actually just increased the primary resistance of the transformer, which has resistance anyway. Nothing ventured, nothing gained. An excellent solution to an irritating problem. You could have done this with an appropriate size capacitor as well, I guess. A lot of cheap Chinese products just employ capacitor droppers. It is illegal in Europe I take it but it is done. Capacitors don't radiate heat either unless broken.
Work it does fortunately. Next time it is out and with the top off I'll have a measure for curiosity. The very small standby transformer is silent, it was the main transformer that hummed.
I did actually measure the primary resistance at 310 ohms and was really surprised that the little cap across the relay contacts is able to maintain around 17 volts across the primary in standby. Perhaps just keeping a little voltage on that unregulated analogue line helps with a silent switch on although I think the muting transistors will be permanently on in standby as the system control is fed from regulated 3.3v rail from the tiny standby transformer.
I recently got a broken Marantz amplifier. Guess what: The main transformer is dead. Thermal fuse is broken, and, even though I had managed to bypass it, it still did not come to life. No solution other than to replace it (or rewind it)
There is just one tap: 230 V
But, it seems that it didn't help...
I just guess what might have happened...
There is just one tap: 230 V
But, it seems that it didn't help...
I just guess what might have happened...
Attachments
That is bad luck 🙁
If the transformer is still dead then something catastrophic must have happened to the transformer because normally they are super rugged and reliable. This is why you should never defeat safety features... suppose there was an intermittent fault with it and what could happen with no thermal fuse. Perhaps an internal short or shorted turns... anything really. Its not worth the risk.
Of course there may be external reasons for it failing such as a short on a secondary (output transistors or PSU bridge etc).
If the transformer is still dead then something catastrophic must have happened to the transformer because normally they are super rugged and reliable. This is why you should never defeat safety features... suppose there was an intermittent fault with it and what could happen with no thermal fuse. Perhaps an internal short or shorted turns... anything really. Its not worth the risk.
Of course there may be external reasons for it failing such as a short on a secondary (output transistors or PSU bridge etc).
The catastrophe is high mains voltage. While it used to hoover in the middle today it can be at the upper limit but just within tolerance for a longer time. Often with a DC component.
Above saturation they get warm quickly.
“Green” will be the end for many classic/vintage transformer based devices. It is just like that.
Above saturation they get warm quickly.
“Green” will be the end for many classic/vintage transformer based devices. It is just like that.
Last edited:
I agree. It's the heat buildup that kills the primary. I think at first isolation fails somewhere, creating a set of shorted windings. Those will get so hot it melts the wire. In the end the winding goes open and the transformer is scrap metal. Rewinding is mostly not feasable, hours and hours of work. Even more because the primary is usually under the secondaries so every winding has to come off and has to be redone.